Like most cells in vivo, many cells are adherent cells, or anchorage-dependent cells; that is, they can metabolize and divide only if they are attached to a surface or substrate. Only cells of the circulatory system (e.g., lymphocytes and red blood cells) grow unattached and suspended in solution in vitro. While many anchorage-dependent cells may grow on glass or synthetic surfaces, these cells often lose their ability to differentiate and respond to hormones. The loss of cellular morphology not only entails a loss of function, but also prevents regenerative power in a longer-term culture system. Longer-term cultivation would however be of great significance, for example, with the use of human cells for tissue culture, and many cells are not available in any quantity. For this reason, such tissue culture dishes are often coated with extracellular matrix components such as collagen or fibronectin. However, the use of xenogenic factors is a clear disadvantage, especially if the cells as such or on a matrix as used for medical treatment of human beings, as it will bring along risks of contamination and may result in adverse reactions in the patient treated.
The failure of cells to grow on such surfaces or keep their abilities is a major limitation of current tissue culture techniques. Tissue cultures are a potential source of tissues and organs which could be used for transplantation into humans. For example, tissue cultured skin cells could potentially be used in skin grafts. The aim is to develop biological substitutes that can restore and maintain normal function, for example, by the use of acellular matrices, which will depend on the body's ability to regenerate for proper orientation and direction of new tissue growth, or by the use of matrices or membranes with cells adhered thereto (Curr. Opin. Pediatr. 16, 167-171). Cells can also be used for therapy via injection, either with carriers or alone. In such cases, the cells need to be expanded in culture, attached to a support matrix, and then re-implanted into the host after expansion.
The ability to culture cells, especially adherent cells, is important also because they represent biological “factories” capable of producing large quantities of bio products such as growth factors, antibodies and viruses. These products can then be isolated from the cell cultures and used, for example, to treat human diseases.
Cell cultures also are emerging tools for biocompatibility and toxicology studies in the field of pharmaceutical and life science industry.
Finally, tissue cultures usually comprise cells from only one or a few tissues or organs. Consequently, cell cultures provide scientists with a system for studying the properties of individual cell types without the complications of working with the entire organism.
A known method for adherent cell cultures involves a hollow fiber membrane bioreactor. In this system, the cells are generally attached to the lumen of a cylindrical hollow fiber membrane. Culture media and oxygen flows through the hollow fiber membrane. The molecular weight cut-off of the membrane permits nutrients and oxygen to reach the cells without allowing the cells to escape.
A variety of polymers has been suggested for producing semipermeable membranes for cell and tissue culture (US 2007/269489). They include polyalginate, polyvinylchloride, polyvinylidenefluoride, polyurethane, cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose nitrate, polysulfone, polyethersulfone, polystyrene, polyurethane, polyvinyl alcohol, polyacrylonitrile, polyamide, polymethylmethacrylate, polytetrafluoroethylene, polyethylene oxide and combinations of such polymers. The polymeric support may also consist of polyethyleneterephthalate (PET) or polycarbonate. Further materials which were suggested, for example, as scaffolds for transplantable tissue material, are cellulose or macroporous collagen carriers, or biodegradable matrices.
Apart from the problem of identifying membranes which could be used as a matrix for the cultivation of adherent cells, membranes currently known in the art suffer from their inability to sufficiently promote and sustain adherence, expansion, differentiation and extended life-span without the pre-treatment of said membranes or matrices, or the addition of exogenous factors, such as, for example, fibronectin, laminin or collagen.
In Expert Rev. Med. Devices 3(2), 155, efforts are reviewed with regard to developing an artificial kidney based on adhering renal tubule cells to a synthetic polysulfone-based hollow-fiber membrane. In this case the membrane has to be coated with ProNectin-L™ in order to promote attachment of the cells.
U.S. Pat. Nos. 6,150,164 A and 6,942,879 B1 both present elaborate ways towards a bio-artificial kidney based on renal cells such as, for example, endothelial cells or so-called renal stem cells, which are seeded into hollow fibers. Hollow fiber membranes which are mentioned as being useful are based on cellulose, polyacrylonitrile, polysulfone and other components or copolymers thereof. The internal or external surface of the hollow fiber is pre-coated with suitable extracellular matrix components (EMC) including Type I collagen, Type IV collagen, laminin, Matrigel, proteoglycan, fibronectin and combinations thereof. Only after such treatment the cells can be seeded.
J. Mat. Sci. 9 (1998) 711 discloses the cultivation of cloned Madin Darby Canine Kidney (MDCK) renal epithelial cells on the surfaces of polysulfone and polyacrylonitrile membranes, respectively. The MDCK cells were able to form epithelial-like layers on the membranes.
WO 02/00775 A1 discloses cultivation of chondrocytes on foam supports made of a copolymer of acrylonitrile and sodium methallylsulfonate (AN69). It is proposed that the foams can also be used for the cultivation of stem cells, keratocytes, hepatocytes, and islets of Langerhans. No mention is made of renal cells.